PRE-ALGEBRAAAA

If 70% of the members remembered the password but 21 members forgot, then how many members were there in all?

m = members

p = 70 %

$\small{ \begin{array}{rcl} m &=& m\cdot p + \underbrace{(1-p)\cdot m}_{=21} \\ m &=& m\cdot p + 21\\\\ m -m\cdot p &=& 21\\\\ m(1- p) &=& 21\\\\ m &=& \frac{ 21} { 1- p} \qquad | \qquad p = 70 \% \\\\ m &=& \frac{ 21} { 1- 70 \%} \\\\ m &=& \frac{ 21} { 100 \%- 70 \%} \\\\ m &=& \frac{ 21} { 30 \%}\\\\ m &=& \frac{ 21} { \frac{30}{100} } \\\\ m &=& \frac{ 21\cdot 100} { 30 } \\\\ m &=& \frac{ 2100 } { 30 } \\\\ m &=& \frac{ 210 } { 3 } \\\\ m &=& 70 \\ \end{array} }$

There  were 70 members in all

Let M be all members, then we have:

.70M + 21 =M

21=M - .70M

21=.30M divide both sides by .30

M=21/.30

M=70-number of all members